In the above devices, electric motors are used to transform electric energy into mechanical energy. Usually electric motors are provided with a rotor secured to the drive shaft and the rotating elements are directly or indirectly coupled to the motor drive shaft. By applying an electric voltage to the motor, a rotary motion is transmitted to the rotor that transfers motion to impellers, pumps, rotors, screw conveyors, blades or equivalent devices. Electric motors are e.g. used for operating the pumps which supply water to a hydraulic circuit, for rotating the impellers of pumps or fans, to operate rotative heat exchanger, mixers etc. In many cases it is required to control and adjust operation of the devices coupled with electric motors by way of monitoring the number of revolutions. In other words, it is often necessary to know the speed of rotation of the element operated by the electric motor. Monitoring of the rotation speed allows for setting a proper adjustment of the operated device or allows for controlling its regular functioning. The reasons which lead to undesired variations in the rotation speed of the operated device may be different. For instance the rotation speed of the rotor of a hydraulic pump, initially set to a reference value, may change due to a variation in the resistance of the related hydraulic circuit, for instance because of a valve along the circuit that is not properly regulated, or due to the build up of scale or debris at the rotor bearings.
Traditional control and adjusting systems make use of speedometers (mechanical, electronic, optical, etc.), encoders, stroboscopes, etc. for sensing the angular velocity of the devices operated by the related electric motor. This type of instruments provides an output signal proportional to the measured value of the angular speed of the shaft to which they are applied.
Traditional control and adjusting systems are difficult to assemble on the rotating parts, complicated to calibrate and in some cases easily damaged; they are cumbersome and, thus, it is necessary to provide room for their assembling on a shaft or on a rotating element. Moreover, their proper functioning has to be periodically verified.
In some applications, e.g. in beverage dispensing machines the pumps operated for circulating water or other alimentary fluids, such as syrups, must have low costs and reduced dimensions. In these machines the water supply to the distribution circuit is usually carried out by a pump which comprises an impeller immersed in a reservoir containing water. When, as it is very usual for this type of pumps, the rotation speed of the impeller decreases due to a build up of scale and deposits within the respective bearings, the amount of fluid, e.g. water or syrup, supplied in a given amount of time (e.g. 10 seconds) by the pump, is lower than the optimal value for the same dispensing time, the consequence being negative changes in the organoleptic characteristics of the beverage.
Therefore there is the need of a system for controlling and adjusting the impeller operation in the aforesaid case and, generally, for controlling and adjusting the operation of a device operated by the motor in order to compensate for undesired variations in the operating parameters, such as angular velocity of the driving shaft, and ensure the correct functioning of the operated device.
JP-A-62213594 discloses a process and a device for smoothly restarting an AC motor after a power interruption that provide to measure the residual voltage of the motor during free rotation and to operate a set switch when said residual voltage value becomes a predetermined value or lower to restart the motor with the help of an inverter. JP-A-6098594 discloses a similar system. JP-A-6070593 also discloses a similar system, further comprising means to control the power feeding when residual voltage is zero.